This project involves the use of mechanical perturbations to study the contractile mechanism of reproductive smooth muscle (uterus and related tissues of rats, rabbits, and guinea pigs). The perturbations, to be delivered during stimulated contractions, will include long stretches and very small sinusoidal length changes. Resulting force responses will be analyzed in real time to determine their amplitude and phase and digitized by a computer. Data will be analyzed to yield force-related long- and short-range dynamic stiffness as determined under a variety of modes of contraction. The ultimate goal of the analysis is to permit making inferences concerning subcellular mechanisms of contraction. The date will be viewed in the context of a cross-bridge and sliding-filament contraction model whose computer-simulated behavior will be compared to actual experimental results. Preliminary work using this approach has successfully reproduced some aspects of the experimental findings. Carefully-defined contractile states will also be produced by using calcium-activated smooth muscle preparations with cell membranes made permeable to small chemical species by the action of non-ionic detergents. Using these as models of partially-activated steady-state conditions, analyses similar to those above will be made and integrated with previous findings. The ultimate aim is to provide a clearer picture of the control and functioning of the contractile process in smooth muscle.